Arrangement for the pulse modulation of a beam of charged particles accelerated by high potentials



Sept. 18, 1962 ARRANGEMENT FOR THE PULSE Filed July 14, 1959 BIAS VOLTAGE GENERATOR OPlTZ 3,054,962 MODULATION OF A BEAM OF CHARGED PARTICLES ACCELERATED BY HIGH POTENTIALS TRANSFORMER 5 SheetsSheet 1 HIGH TENS/0N 8 GENERATOR PULSER F/LAME/VT CURRENT GENERATOR ISOLAT/NG 5 TRANSFORMER 32 l 30 g 33 g: M T x awn/7w w. OPITZ 3,054,962 T FOR THE PULSE MODULATION OF A BE OF CHARGED ARTICLES ACCELERATED BY HIGH POTEN LS 3 Sheets-Sheet 2 M M 2% l %M 00 m 1 J t. w w l l S F Sept. 18, 1962 ARRANGEMENT F W. OPITZ OR THE PULSE MODULATION OF A BEAM OF CHARGED PARTICLES ACCELERATED BY HIGH POTENTIALS Filed July 14, 1959 +5oov 3 Sheets-Sheet 5 B/AS VOL TAGE GENERATOR Ky *M 3,054,962 Patented Sept. 18, 1962 3,054,962 ARRANGEMENT FGR THE PULSE MODULATION OF A BEAM (BF CHARGED PARTICLES ACCEL- ERATED BY HIGH PGTENTIALS Wolfgang @pitz, Aalen, Wurttemberg, Germany, assignor to Carl Zeiss, Wurtternberg, Germany Filed July 14, 1959, Ser. No. 826,968 Claims priority, application Germany July 14, 1958 2 Claims. (Cl. 3282Z7) The present invention relates to an arrangement for the pulse modulation of a beam of charged particles accelerated by high potentials.

It is necessary to modulate the beam of charged particles, in many applications, such as electron-beam devices, electron microscopes, high voltage Oscilloscopes, particle injector systems of linear and circular accelerators, watercooled X-ray tubes, X-ray flash tubes, transmitting tubes of high operating voltage with water-cooled plates and pulse-controlled field-emission cathodes.

In beam generators employed in such applications, it is customary to ground the anode or plate and any elements beyond the anode along the further path of the beam. Grounding of the anode requires application of the accelerating potential to the cathode and the control electrode of the beam generating system. Thus, both the cathode and the control electrode are normally biased to a highvoltage potential. The auxiliary voltages necessary for operation of the beam-generating system, such as cathode heater voltage and control electrode bias voltage, must therefore be produced at high-voltage potential. In addition, if pulse modulation of the beam is to be effected by modulated control signals applied to the cathode or control electrode, then the modulation voltage must either be produced at high-voltage potential of the associated electrode bias or produced at ground potential and transferred via a suitable circuit element to high-voltage potential.

For the pulse modulation of a beam of charged particles, it is known to produce control pulses by an electronic pulse generator which is at a high-voltage potential. Power may be supplied to the generator, for example, from the power lines via an isolating transformer. This arrangement has the disadvantage that a large, complicated electronic apparatus must be arranged at a sufiiciently protected place. A grounded jacket enclosing the generator for protection against electric shock due to contact therewith will cause the generator to become excessively large and unwieldy. Furthermore, protection of the pulse generator from surge voltages and steep wave fronts can be obtained only with great difliculty in this known arrangement.

It is also known to transfer a pulse voltage produced at ground potential to high-voltage potential via a high-voltage capacitor of sufficiently large capacitance. In this known apparatus, protection of the pulse generator from surge voltages is difficult to obtain.

The arrangement in accordance with the present invention permits the pulse modulation of a beam of charged particles of high accelerating voltage produced by means of a beam generating system with grounded anode, while avoiding the indicated disadvantages of the known arrangements.

In accordance with the present invention, the electrode serving to control the beam generating system is connected to the secondary winding of a wide-band isolating transformer which winding is at the high-voltage potential of said electrode, its primary winding being connected with a pulse generator which is at ground potential.

In particular, the object of the present invention is to provide an arrangement which permits a pulse modulation of a beam of charged particles accelerated by high potentials in which the duration of the pulse can be varied within a wide range.

In order to obtain the largest possible band width of the isolating transformer, the latter is advantageously formed of a toroidal core developed from thin tape of magnetic core material having a high permeability and high saturation inductance on which is wound a first single layer winding. The core and first winding are embedded in insulating potted resin in a toroid overlying this winding and concentric thereto. A second single-layer winding is Wound on the potted resin toroid. The transformer turns ratio is 1:1.

The inductance of the isolating transformer should be as great as possible, while its leakage inductance and capacitance should be as small as possible in order to be able to obtain a large band width. The large insulated spacing between windings reduces the interwinding capacitance. The leakage inductance is, however, large for the same reason. Upon an increase in the number of turns, the inductance increases, it is true, but the leakage inductance increases in the same proportion.

The influence of the leakage inductance on the band width can, however, be excluded by the development of the primary and secondary windings as single-layer concentric toroids and by the selection of the transformer turns ratio as 1:1. The isolating transformer in this event, will show the same transient response to a step voltage such as occur for instance on the pulse flanks, as a delay line and the upper limit of the transmitted band width is determined for all practical purposes only by the capacitances. If the toroidal core of the isolating transformer is developed as toroidal ribbon core, of fine lamination, high permeability and high saturation induction, a high inductance of the transformer can be obtained. By this measure, the result is obtained that the lower limit of the transmitted band width is relatively low.

Since the controlling of an electrode of the beam generating system does not bring about any ohmic loading of the isolating transformer, the rear pulse flank will be smoothed off upon a transmission of pulses if no further measures are taken. In order to avoid this effect, the isolating transformer is damped on the secondary side.

It is advisable in known manner when using a Wehnelt electrode in the beam generating system to employ it as the pulse control electrode and to connect a rectifier serving to produce the control electrode bias voltage and lying at high-voltage potential in series with the secondary winding of the isolating transformer.

Upon the pulse modulation of a beam of charged particles, it is advantageous to make the control electrode bias voltage generator adjustable in order to be able to change the amplitude of the beam pulses. One advantageous circuit of a pulse generator for producing the control pulses consists of a multi-vibrator, followed in series by a pulse modulator and of a single shot triggered multivi'orator controlled by said pulse modulator and a power output stage. In this connection, it is advisable to develop one stage of the single shot multi-vibrator as driver tube. By this measure, a further preamplification stage is saved so that therefore the flank slope of the control pulses pro-;

duced by said multi-vibrator is retained without further measures.

Furthermore, it has proven advantageous to connect the pulse modulator via decoupling elements directly with a grid of the driver tube. In this way the result is obtained that the pulse of steep flank supplied by the pulse modulator causes an immediate commencement of the flow of current through the driver tube.

For the transmission of the control pulses taken from the anode of the driver tube to the power output stage,-

there is used a pulse reversal transformer which advisedly has the same transmission properties as the isolating transformer serving to transmit the control pulses to the control electrode of the beam generating system. In this way the result is obtained that pulses which are properly trans mitted by the isolating transformer are not smoothed down before being fed to said transformer.

The invention will be explained in further detail with reference to the accompanying drawings of which:

FIGURE 1 is a schematic diagram of an arrangement in accordance with the invention as applied to the pulse modulation of an electron beam;

FIGURE 2 is a plot of the voltage waveform at the control electrode of the beam generating system in which voltage is plotted along the scale of ordinates and time is plotted along the scale of abscissa;

FIGURE 3 is a partially sectioned elevation of the pulse isolating transformer used in the circuit of FIG- URE 1;

FIGURE 4 is a section along the plane IVIV of FIGURE 3;

FIGURE 5 is a partially sectioned elevation of a beam generator in accordance with this invention;

FIGURE 6 is a schematic diagram of a suitable pulse generator circuit for the generating of control pulses.

In FIGURE 1 there is shown apparatus for generation of a beam of charged particles, specifically an electron beam generator 1 the beam producing system of which comprises a cathode 2, a control electrode 3 and a grounded anode 4. The electron beam produced by this beam generating system is marked 5. The cathode heater voltage is supplied by source 6 maintained at the high bias potential of the cathode. The cathode is biased to the desired high-voltage potential (for example -100 kv.), by source 8 through a protective dropping resistor 9 in applications where such protection is desirable. To bias the control electrode 3 with respect to the cathode 2 there is provided a direct voltage generator 12 coupled between the cathode and control electrode. Alternating voltage is applied to this generator by an isolating transformer 13. The generator 12 supplies a voltage of, for example, one kilovolt so that the control electrode 3 has a negative bias of 1 kv. with regard to the cathode 2. Control of the amplitude of the bias of the control electrode is conveniently afforded by controlling the amplitude of the alternating voltage applied to the primary of transformer 13.

In FIGURE 1, all elements above the line 18 are at high voltage, while all elements below line 18, namely the anode 4, the pulse generator 10, the primary winding of the transformers 10, 13 are at ground potential as opposed to the high biasing potentials. For example, housing of generator 10 is at ground potential as opposed to the biasing potential, although the enclosed elements are at the normal operating potentials.

For the modulation of the electron beam 5, there is provided a pulse generator 19 lying at ground potential and connected with the primary winding of an isolating transformer 11 The core and the secondary winding of this isolating transformer lie at high voltage, the secondary winding being serially connected with generator 12 between the cathode 2 and control electrode 3.

In FIGURE 2 there is shown a plot of the waveform of the control electrode potential with the reference axis corresponding to the bias potential of the control electrode. Line 14 parallel to the reference axis represents the potential of the cathode with respect to the control electrode. The cut-off potential, i.e. the relative potential of the cathode with respect to the control electrodes potential to cut off generation of a beam of charged particles, is represented by line 16. Thus, the beam generating system is normally blocked. If a pulse voltage 17 is now applied through the isolating transformer 11, the voltage at the control electrode 3 will be raised to the working voltage for the duration of each pulse. This working voltage lies above the cut-off voltage of the system so that therefore an electron beam pulse is produced.

4 It will be noted that with a pulse of fixed amplitude, variation in the beam pulse amplitude can be controlled by adjustment of the control electrode bias voltage.

The isolating transformer 11 is shown in FIGURES 3 and 4 and comprises a toroidal core 19 which is advantageously developed as a toroidal ribbon core of thin lamination and high saturation induction. The core 19 is embedded in a protective layer 20 of porous soft material, for instance foam material. Over this protective layer, there is located the winding 21 developed in a single layer. The core 19 and winding 21 are embedded in insulating potted resin in an overlying toroid 22. By the shrinkage of said resin upon the hardening, mechanical stresses occur which can considerably change the magnetic properties of the core 19. In order to prevent this effect, the core is embedded in the said protective layer 20. On the potted resin toroid 22 is wound a secondary winding 23 which is also developed in a single layer and can be fixed in position by a coating of cold-hardening resin, or by a layer of lacquer.

In order to make possible the feeding of current to the inner winding 21 of the isolating transformer while maintaining the necessary insulation paths, the latter is provided with an insulator projector 24 which is potted together with the potted resin toroid 22. The feeds 25 and 26 extend through the insulation extension 24 to the inner winding 21.

In FIGURE 5, the mechanical construction of the arrangement in accordance with the invention is shown. As can be noted from this figure, the isolating transformer 11 is so connected with an oil-filled housing 27 that its insulation extension 24 extends into said housing. A shielded 3-wire high voltage cable 28 is also connected with the housing 27, the heating voltage which lies at high voltage as well as the control electrode bias voltage being fed via said cable. The control electrode bias voltage passes from the high voltage cable 28 through the secondary winding of the isolating transformer 11 and through an insulator 29 to the control electrode 32. The cathode heater voltage is fed from cable 28 directly through insulator 29 to the cathode 33. The control pulses generated by the generator 10 are fed to the primary winding of the isolating transformer 11.

The beam generating system is arranged in a grounded housing 30 which is under a high vacuum. The anode 34- of the beam generating system is also grounded.

FIGURE 6 shows the basic circuit diagram of a pulse generator for generating square pulses of variable duration and pulse repetition rate and high flank slope. The generator comprises a conventional multi-vibrator 35. The frequency of the pulses supplied by this multi-vibrator can be regulated by selection of suitable coupling capacitance. The pulses generated are fed via a differentiator 36, 52 to the control grid of a thyratron 37 connected as pulse modulator. Parallel to 37, there is connected a capacitor 40 which discharges via the fired thyratron, whereby a pulse of short duration and great flank steepness is produced.

This control pulse of steep sides is fed via a recitfier 41, serving as a stage decoupler, directly to the control grid of a driver tube 42. The tube 42 is combined with another electronic tube 43 to form a conventional cathodecoupled, single-shot, triggered multivibrator. Selection of the coupling capacitance in the single shot triggered multi-vibrator will regulate the pulse width. Negative pulses of an amplitude of, for instance, 300 v. are taken from the anode of the driver tube 42. These pulses are transmitted via a polarity reversing transformer 46, the transformer ratio of which is 1:-1 to the control grid of the power output stage 47. Negative pulses of an amplitude of, for instance, 1 kv. can then be obtained from the anode of the tube 47. These pulses are fed to the primary winding 48 of the isolating transformer. All circuit elements lying to the right of line 51 with the exception of the housing 1 and the grounded anode 4 are aoeaaea a high-voltage potential. The isolating transformer has a transformer ratio of 1:1 so that positive control pulses of an amplitude of 1 kv. are produced at the secondary winding 49. The isolating transformer is damped on its secondary side by means of a resistor 50 in order to prevent a smoothing-out of the rear side of the pulse. The control pulses are fed directly to the control electrode 3 of the beam-generating system of an electron beam apparatus. 1.

The transformers 46 and 11, as can be noted from FIG- URE 6, are damped on both side by the circuit elements and the resistor 50 respectively. This damping is necessary since a dependable pulse transmission is possible only if the output is not of too high an ohmic resistance. The power loss necessary for this must be supplied, in addition, by the pulse generator. The pulse generator is so designated that it produces pulses having steep sides and pulse width ranging between 1 and 100 microseconds which can be transmitted excellently over the isolating transformers 46 and 11. The windings of both transformers are of a single layer, and both transformers have a transformer ratio of 1:-1 so that therefore the polarity reversal transformer 46 also behave-s similar to a delay circuit for the pulse sides.

For the protection of the pulse generator against damage from voltage surges and steep wave fronts transmitted to the primary from sparking, flash-over or rapid load change in the high potential secondary winding, it is advisable to coat the primary Winding 23 with a conductive coating of not too high a conductivity.

In one circuit of the pulse generator which was tried out, a voltage of 100 volts was applied to the terminal 38, a voltage of 500 volts to the terminal 53, a voltage of 200 volts to the terminal 44, a voltage of 1500* volts to the terminal 45 and a voltage of -20 volts to the terminal 39. With such a circuit, control pulses of an amplitude of about 1 kv. can be generated.

By developing the tube 42 of the single shot multivibrator directly as driver stage, a further amplification stage can be saved, whereby the steepness of the sides of the control pulses produced is retained without further measures.

The circuit of the pulse generator shown in FIG- URE 6 has been found suitable, but within the scope of the invention any pulse generator can be used which is capable of producing control pulses of steep sides of variable duration and frequency.

I claim:

1. In combination with apparatus for working material by a beam of charged particles which apparatus comprises a cathode, a control electrode, a grounded anode, and a source of unidirectional, fixed amplitude, high voltage coupled to said cathode and said control electrode, means for the pulse modulation of the beam of charge particles produced in said apparatus, said means consisting of a source of unidirectional, variable amplitude, biasing voltage coupled between said cathode and said control electrode for biasing said control electrode with respect to said cathode in such a way that said beam is cut ofi, a wide band isolating transformer having a primary and a secondary winding, said secondary winding being coupled serially between said bias voltage generator and said control electrode, and a control pulse generator at ground potential coupled across said primary winding, said control pulse generator producing pulses of predetermined fixed amplitude to overcome the bias of the control electrode and to produce an electron beam during the duration of each control pulse.

2. The combination in accordance with claim 1 in which said isolating transformer comprises a toroidal core of thin laminations, a single-layer secondary winding wound on said core, said core and winding being embedded in insulating potted resin in an overlying toroid, a second single-layer primary winding wound on said potted resin in a concentric toroid, the turns ratio between said windings being 1:1.

References Cited in the file of this patent UNITED STATES PATENTS 2,392,380 Varian Jan. 8, 1946 2,482,768 Hansen et a1 Sept. 27, 1949 2,569,827 Paulsen Oct. 2, 1951 2,735,074 McArthur Feb. 14, 1956 2,748,316 Stevenson May 29, 1956 2,773,183 Gund et al Dec. 4, 1956 2,815,445 Young et al. Dec. 3, 1957 2,820,142 Kelliher Jan. 14, 1958 2,823,362 Geroulo et al. Feb. 11, 1958 2,912,616 Marchese et al. Nov. 10, 1959 2,930,011 Wigert et al. Mar. 22, 1960 

